An electroacoustic transducer has reduced loss due to acoustic waves emitted in the transverse direction. For this purpose, a transducer comprises a central excitation area, inner edge areas flanking the central excitation area, outer edge areas flanking the inner edge areas, and areas of the busbar flanking the outer edge areas. The longitudinal speed of the areas can be set so that the excitation profile of a piston mode is obtained.
|
30. An electroacoustic transducer arranged in an acoustic track, the electroacoustic transducer comprising:
a piezoelectric substrate;
two busbars arranged on the substrate;
two electrodes arranged on the substrate, each electrode having interdigital electrode fingers interconnected with a respective one of the two busbars, the electrodes configured for the excitation of an acoustic wave;
a plurality of regions running parallel to the acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in each of the regions, the regions comprising:
a central excitation region with a first longitudinal velocity,
outer edge regions flanking inner edge regions and the central excitation region on both sides, wherein the longitudinal velocity in the outer edge regions deviates from the longitudinal velocity of the central excitation region and wherein the longitudinal velocity is higher in the inner edge regions than in the central excitation region;
regions of the busbars flanking the outer edge regions, wherein the longitudinal velocity in the regions of the busbars is lower than the longitudinal velocity in the outer edge regions;
wherein kx2+(1+Γ)ky2=k02 and Γ<−1; and
wherein kx is a component of a wave vector in a longitudinal direction, Γ is an anisotropy factor, ky is a component of the wave vector in a transverse direction, and k0 is the wave vector in a main propagation direction.
1. An electroacoustic transducer arranged in an acoustic track, the electroacoustic transducer comprising:
a piezoelectric substrate;
two busbars arranged on the substrate;
two electrodes arranged on the substrate, each electrode having interdigital electrode fingers interconnected with a respective one of the two busbars, the electrodes configured for the excitation of an acoustic wave; and
a plurality of regions running parallel to the acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in each of the regions, the regions comprising:
a central excitation region with a first longitudinal velocity;
inner edge regions flanking the central excitation region on both sides, wherein the longitudinal velocity in the inner edge regions deviates from the longitudinal velocity of the central excitation region;
outer edge regions flanking the inner edge regions, wherein the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions or is higher than the longitudinal velocity in the central excitation region;
regions of the busbars flanking the outer edge regions, wherein the longitudinal velocity in the regions of the busbars is lower than the longitudinal velocity in the outer edge regions;
wherein kx2+(1+Γ)ky2=k02 and Γ>−1, when the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions;
wherein and kx2+(1+Γ)ky2=k02 and Γ<−1, when the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the central excitation region;
wherein the longitudinal velocity is higher in the inner edge region than in the central excitation region; and
wherein kx is a component of a wave vector in a longitudinal direction, Γ is an anisotropy factor, ky is a component of the wave vector in a transverse direction, and k0 is the wave vector in a main propagation direction.
31. An electroacoustic transducer arranged in an acoustic track, the electroacoustic transducer comprising:
a piezoelectric substrate;
two busbars arranged on the substrate;
two electrodes arranged on the substrate, each electrode having interdigital electrode fingers interconnected with a respective one of the two busbars, the electrodes configured for the excitation of an acoustic wave;
a material that is one of hafnium oxide or tantalum oxide and that is disposed on or between the electrode fingers;
a plurality of regions running parallel to the acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in each of the regions, the regions comprising:
a central excitation region with a first longitudinal velocity;
inner edge regions flanking the central excitation region on both sides, wherein the longitudinal velocity in the inner edge regions deviates from the longitudinal velocity of the central excitation region;
outer edge regions flanking the inner edge regions, wherein the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions or is higher than the longitudinal velocity in the central excitation region;
regions of the busbars flanking the outer edge regions, wherein the longitudinal velocity in the regions of the busbars is lower than the longitudinal velocity in the outer edge regions;
wherein kx2+(1+Γ)ky2=k02 and Γ<−1, when the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions;
wherein kx2+(1+Γ)ky2=k02 and Γ<−1, when the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the central excitation region; and
wherein kx is a component of a wave vector in a longitudinal direction, Γ is an anisotropy factor, ky is a component of the wave vector in a transverse direction, and k0 is the wave vector in a main propagation direction.
2. The electroacoustic transducer as claimed in
3. The electroacoustic transducer according to
4. The electroacoustic transducer as claimed in
5. The electroacoustic transducer as claimed in
6. The electroacoustic transducer as claimed in
7. The electroacoustic transducer as claimed in
8. The electroacoustic transducer as claimed in
9. The electroacoustic transducer as claimed in
10. The electroacoustic transducer as claimed in
11. The electroacoustic transducer as claimed in
12. The electroacoustic transducer as claimed in
13. The electroacoustic transducer as claimed in
14. The electroacoustic transducer as claimed in
15. The electroacoustic transducer as claimed in
16. The electroacoustic transducer as claimed in
17. The electroacoustic transducer as claimed in
18. The electroacoustic transducer as claimed in
the longitudinal velocity in each of the inner edge regions is identical;
the longitudinal velocity in each of the outer edge regions is identical; and
the longitudinal velocity in each of the regions of the busbars is identical.
19. The electroacoustic transducer as claimed in
wherein the normalized overlap integral is given by:
<Φ|Ψ>/√{square root over (<Φ|Φ><Ψ|Ψ>)}≦1; where (1) is a transverse excitation profile and Ψ is a transverse deflection profile.
20. The electroacoustic transducer as claimed in
21. The electroacoustic transducer as claimed in
22. The electroacoustic transducer as claimed in
23. The electroacoustic transducer as claimed in
24. The electroacoustic transducer as claimed in
25. The electroacoustic transducer as claimed in
26. The electroacoustic transducer as claimed in
27. The electroacoustic transducer as claimed in
28. The electroacoustic transducer as claimed in
29. A method for producing an electroacoustic transducer in an acoustic track as claimed in
providing a piezoelectric substrate;
providing busbars on the substrate;
providing two electrodes arranged on the substrate, each electrode comprising at least one of hafnium or tantalum and having interdigital electrode fingers interconnected with a respective one the busbars, the electrodes configured for excitation of an acoustic wave; and
oxidizing material of the at least one of hafnium or tantalum of the electrode fingers electrodes in an inner edge region;
wherein a plurality of regions run parallel to the acoustic track, wherein the acoustic wave experiences different longitudinal propagation velocity in different ones of the plurality of regions;
wherein the plurality of regions comprises:
a central excitation region, the central excitation region having a longitudinal velocity;
the inner edge regions, which flank the central excitation region on both sides, wherein a longitudinal velocity in the inner edge regions deviates from the longitudinal velocity of the central excitation region;
outer edge regions flanking the inner edge regions, wherein a longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions or is higher than the longitudinal velocity in the central excitation region; and
regions of the busbars flanking the outer edge regions, wherein the longitudinal velocity in the regions of the busbars is lower than the longitudinal velocity in the outer edge regions;
wherein kx2+(1+Γ)ky2=k02 and Γ<−1, when the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions;
wherein kx2+(1+Γ)ky2=k02 and Γ<−1, when the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the central excitation region; and
wherein kx is a component of a wave vector in a longitudinal direction, Γ is an anisotropy factor, ky is a component of the wave vector in a transverse direction, and k0 is the wave vector in a main propagation direction.
|
This patent application is a national phase filing under section 371 of PCT/EP2010/063562, filed Sep. 15, 2010, which claims the priority of German patent application 10 2010 005 596.4, filed Jan. 25, 2010, each of which is incorporated herein by reference in its entirety.
The invention relates to electroacoustic transducers which find application for example in a SAW or GBAW RF filter, and to methods for producing such transducers. Transducers according to the invention have lower losses due to a reduced transverse emission of acoustic waves and an improved performance due to suppression of transverse modes.
Components that operate with acoustic waves—e.g., surface acoustic waves (SAW) or guided bulk acoustic waves (GBAW)—convert RF signals into acoustic waves and conversely acoustic waves into RF signals. For this purpose, SAW or GBAW components comprise electrode fingers arranged on a piezoelectric substrate or on a piezoelectric layer. In a longitudinal direction, i.e., in the direction in which the acoustic waves propagate, electrode fingers are arranged alongside one another, which are generally connected alternately to a first and a second busbar. The acoustic track is that region of the substrate or of the piezoelectric layer in which surface acoustic waves propagate during the operation of the component. The electrode fingers lie in the acoustic track and thus in the acoustic region. The busbars lie in the lateral edge region of the acoustic track. In a longitudinal direction, the acoustic track is generally delimited by reflectors in order to reduce the energy loss due to emission of the acoustic waves in a longitudinal direction.
One loss mechanism in the case of components that operate with acoustic waves consists in acoustic waves leaving the acoustic track in a longitudinal or transverse direction.
In particular due to the finite aperture of the acoustic tracks, transverse acoustic modes can arise due to diffraction effects. Such modes disturb the transmission characteristic and constitute a loss mechanism. One important point in the development of components that operate with acoustic waves, in particular surface acoustic wave filters for mobile radio applications, is to obtain components having low loss mechanisms—e.g., without disturbing transverse modes or with reduced disturbing transverse modes—in conjunction with a good transmission characteristic.
The published German patent application DE 103 31 323 A1 discloses transducers which operate with SAWs and in which the losses due to transverse oscillations are reduced by cutouts being arranged in the busbars.
The U.S. Pat. No. 7,576,471 discloses components which operate with SAWs and in which the thickness of the electrode fingers is increased in a region between a central excitation region (“center region”) and the region of the bulbar (“busbar region”). In this case, however, the application is restricted to so-called “weakly coupling” substrates. The electroacoustic coupling constant k2 is a measure of the strength of the coupling between acoustic waves and RF signals.
Summary
In one aspect, the present invention specifies an electroacoustic transducer which has low transverse losses and which is compatible with strongly coupling piezoelectric substrates.
In a first embodiment, the invention specifies an electroacoustic transducer arranged in an acoustic track. The transducer comprises a piezoelectric substrate and two electrodes arranged thereon and each having interdigital electrode fingers interconnected with a bulbar, for the excitation of acoustic waves. The transducer is designed such that the acoustic wave in a plurality of regions running parallel to the acoustic track has a different longitudinal propagation velocity. The longitudinal propagation velocity is the velocity of the acoustic wave in a longitudinal direction.
The transducer comprises a central excitation region with a first longitudinal velocity. Inner edge regions flank the central excitation region on both sides, in which inner edge regions the longitudinal velocity deviates from the longitudinal velocity in the central excitation region. Outer edge regions flank the inner edge regions. In the outer edge regions the longitudinal velocity is higher than in the inner edge regions. The outer edge regions can serve for waveguiding. Their width is then large enough to achieve waveguiding—e.g., a decay of the bound modes to zero. Regions of the busbars flank the outer edge regions of the electroacoustic transducer. In the regions of the busbars of the electroacoustic transducer the longitudinal velocity is lower than in the outer edge regions. The substrate has a convex slowness. The slowness is the reciprocal of the velocity. The slowness is proportional to the wave vector k of the acoustic waves propagating in the substrate. The presence of a convex slowness is equivalent to an anisotropy factor Γ of the substrate that is greater than −1: Γ>−1. In this case, the anisotropy factor is defined by the equation
kx2+(1+Γ)ky2=k02
where kx is the component of the wave vector in a longitudinal direction, ky is the component of the wave vector in a transverse direction, and k0 is the wave number in the main propagation direction of the acoustic waves. The main propagation direction in a longitudinal direction x is given by the arrangement of the electrode fingers. The main propagation direction runs perpendicular to the electrode fingers. The abovementioned equation in this case holds true approximately for ky/kx<<1.
In one variant of the first embodiment, the longitudinal velocity is lower in the inner edge region than in the central excitation region.
In one embodiment, the longitudinal velocity in the region of the busbars is lower than the longitudinal velocity in the inner edge regions.
In one embodiment, the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the central excitation region.
In another embodiment, the invention specifies an electroacoustic transducer arranged in an acoustic track. The transducer comprises a piezoelectric substrate and two electrodes arranged thereon and each having interdigital electrode fingers interconnected with a busbar, for the excitation of acoustic waves. The transducer is designed such that the acoustic wave in a plurality of regions running parallel to the acoustic track has a different longitudinal propagation velocity.
The transducer comprises a central excitation region with a first longitudinal velocity. Inner edge regions flank the central excitation region on both sides, in which inner edge regions the longitudinal velocity deviates from the longitudinal velocity in the central excitation region. Outer edge regions flank the inner edge regions. In the outer edge regions the longitudinal velocity is higher than in the central excitation region. Regions of the busbars flank the outer edge regions of the electroacoustic transducer. In the regions of the busbars of the electroacoustic transducer the longitudinal velocity is lower than in the outer edge regions.
The regions of the busbars are of a size such that wave guiding is possible in these regions. In contrast to the first variant, the region of the busbars serves for waveguiding.
The substrate has a concave slowness. The presence of a concave slowness is equivalent to an anisotropy factor Γ of the substrate that is less than −1: Γ<−1.
In one variant of the second embodiment, the longitudinal velocity is higher in the inner edge region than in the central excitation region.
In one embodiment, the longitudinal velocity in the outer edge regions is higher than the longitudinal velocity in the inner edge regions.
In one embodiment, the longitudinal velocity in the regions of the busbars is lower than the longitudinal velocity in the central excitation region.
Such configurations of an electroacoustic transducer in which the longitudinal velocity varies in a transverse direction produce a transverse profile of the longitudinal velocity in which a so-called “piston mode” is capable of propagation. The piston mode is an oscillation mode characterized by the fact that the profile of the maximum deflection of the atoms of the piezoelectric material is substantially constant within the excitation region and preferably zero in the region outside the acoustic track. In between, the maximum deflection decreases with a gradient as high as possible. Quantitatively, a “good” piston mode is characterized in that the overlap integral of the fundamental mode:
∫|Φ(y)Ψ(y)|dy
formed from transverse excitation profile Φ(y) and transverse deflection profile Ψ(y) is as large as possible. Another notation for this integral is: <Φ|Ψ>.
The piston mode is furthermore characterized by the fact that no or at most minimal acoustic waves propagating in a transverse direction occur. Achieving the piston mode is therefore an effective means for reducing energy losses due to transverse emission of acoustic waves from the acoustic track and at the same time for achieving an improved performance due to suppression of transverse modes.
The transducer described above enables an improved piston mode compared with known transducer structures, i.e., an enlarged overlap integral of the fundamental mode. Furthermore, the transducer according to the invention is compatible with highly coupling substrates.
The setting of the longitudinal velocity in regions of the acoustic track which are arranged transversely alongside the central excitation region is essential for achieving a high value of the overlap integral.
The subdivision of the region of the acoustic track between the central excitation region and the region of the busbars into regions having a different longitudinal velocity makes it possible to obtain a piston mode with a flank region that can be set better. In particular, the gradient of the deflection function is increased.
In one embodiment, the busbar and the electrode fingers are arranged on a piezoelectric substrate having a higher electroacoustic coupling coefficient than quartz. By way of example, lithium tantalate or lithium niobate is appropriate as such a piezoelectric substrate.
The designation “concave” slowness relates to the ratio of ky, the wave number in a transverse direction, to kx, the wave number in a longitudinal direction. A concave slowness means that the slowness in a transverse direction, which is proportional to ky, as a function of the slowness in a longitudinal direction, which is proportional to kx, is a concave function: the second derivative of the slowness in a longitudinal direction with respect to the slowness in a transverse direction is positive. Alternatively the following equivalently holds true: the second derivative of kx with respect to ky is positive:
A piezoelectric substrate having concave slowness has a focusing effect on acoustic waves and thereby helps to reduce the emission of acoustic waves in a transverse direction.
In one embodiment, the electrode fingers at least in sections along the transverse direction are wider within the inner edge regions than in the central excitation region.
In one embodiment, the electrode fingers at least in sections along the transverse direction are narrower within the inner edge regions than in the central excitation region.
The velocity of the acoustic waves at the surface of a piezoelectric substrate is dependent on the mass covering of the substrate, i.e., on the mass of the layers that are arranged on the substrate. Materials of the electrode fingers constitute such layers. In this case, an acoustic wave is all the slower, the higher the mass covering, and all the faster, the greater the elastic constants of the material of the mass covering. Widened electrode fingers generally constitute an increased mass covering. A finger widening restricted to the inner edge region is thus a simple but effective means for reducing the longitudinal velocity in the inner edge region. Depending on the material, a mass covering (e.g., with Al2O3 or diamond, both materials are relatively light but have high stiffness values) can also increase the velocity.
Likewise, a narrowing or widening of the electrode fingers can bring about a decrease or an increase in the velocities.
In one embodiment, the width of the electrode fingers at least in sections along the transverse direction changes linearly within the inner edge regions. The fact that the finger width changes linearly, i.e., not in a stepped fashion, provides a further degree of freedom in the shaping of the deflection profile and thus of the piston mode. In this case, the width can increase or decrease from the inner area outward.
In one embodiment, the electrode fingers at least in sections along the transverse direction are higher or lower within the inner edge regions than in the central excitation region. A thickening or thinning of the fingers likewise constitutes one possibility for altering the mass covering in order to obtain an improved piston mode.
In one embodiment, the electrode fingers are higher in the central excitation region than in the inner edge regions, the outer edge regions or the regions of the busbars and in this case have, in particular, a thicker metallization. A thickening and also a thinning, depending on material parameters, of the fingers likewise constitute one possibility for altering the mass covering. By setting the thickness of the fingers, the acoustic velocity can easily be set in order to obtain an improved piston mode.
In one embodiment, the height of the electrode fingers, i.e., the thickness of the electrode layer on the substrate, within the inner edge regions changes in a stepwise manner at least in sections along the transverse direction.
Electrode fingers and busbars are usually applied on a piezoelectric substrate in deposition processes (for example, using lift-off technology or using etching technology). In this case, a linear change in the thickness cannot be realized in a trivial manner. In the case of a stepwise change it is possible, if the step size is chosen to be small enough, to choose and obtain a good approximation to a linear profile.
In one configuration, the height of the electrode fingers within the inner edge regions increases linearly at least in sections along the transverse direction, i.e., outwardly or inwardly. If an approximation by a thickness changed in a stepped fashion is not sufficient, then a linearly or otherwise continuous function of the layer thickness can be obtained by virtue of the fact that, during the deposition process, the material jet has a spatially inhomogeneous flow rate and the deposition rate differs in different regions of the substrate. The gradient of the deposition rate is then a spatially continuous function.
In one embodiment, a conductive or dielectric material different than the electrode material is arranged on the electrode fingers in the inner edge regions at least in lateral sections. Such a material arranged on the electrode fingers furthermore makes it possible to set the velocity of the acoustic wave on account of the different mass covering.
In one embodiment of a transducer, dielectric material is arranged in the inner edge regions on and between the electrode fingers. It is possible as it were to lay a rail, e.g., structured by means of lift-off technology or by means of etching technology, in a longitudinal direction over the electrode fingers. As a result, the mass covering and thus the longitudinal velocity can easily be set.
In one embodiment, hafnium oxide or tantalum oxide is arranged on or between the electrode fingers.
Hafnium oxide and tantalum oxide are compounds having a high specific density and thus have a great influence on the change in the velocity of the acoustic wave. In addition, they are electrical insulators, such that different fingers having a different polarity are not short-circuited.
Hafnium or tantalum, i.e., the metals themselves, can also serve to reduce the velocity. For this purpose, they are arranged on the electrodes, the busbars, on stub fingers or the electrode fingers.
In this case, the propagation velocity can be set such that a focusing in the propagation direction occurs on account of the anisotropy.
In one embodiment, the longitudinal velocity is higher in the outer edge regions than in the central excitation region.
In one embodiment of the electroacoustic transducer, the longitudinal velocity is higher in the inner edge regions than in the regions of the busbars.
In one embodiment, the longitudinal velocity is identical in the two inner edge regions. The longitudinal velocity in the two outer edge regions is also identical in each case here. The longitudinal velocity is identical in the regions of the two busbars.
In one embodiment, the match of the excitation profile of the fundamental mode of the acoustic wave in a transverse direction or of the transducer and of the deflection profile of the acoustic wave in a transverse direction—the overlap integral—is as high as possible. The normalized overlap integral is preferably greater than 0.9 or greater than 0.95 or greater than 0.99. In this case, the normalized overlap integral is:
In one embodiment, the transverse excitation profile of the fundamental mode of the acoustic wave is adapted to the transverse deflection profile by a phase weighting in the inner edge region. In this case, a phase weighting is obtained by individual regions, for example regions arranged in a transverse direction, of the electrode fingers having an excitation center—generally the center of the electrode fingers—which is displaced in a longitudinal direction relative to the excitation center in other regions. Such a displacement can be achieved by the widening or the narrowing of the electrode fingers not being implemented symmetrically with respect to the finger center in a longitudinal direction.
By means of a displacement of the excitation center of parts of the electrode fingers, what is achieved, on account of the mismatch with respect to the otherwise sharply defined orientation of the acoustic wave, its wavelength and the excitation center, is that the excitation intensity is reduced in these lateral regions. The developer of an electroacoustic transducer is thus provided with a further degree of freedom for setting the excitation profile and thus for setting a piston mode.
In one embodiment, the electrode fingers or the busbars are covered with a dielectric layer. In one configuration of this electroacoustic transducer, the layer consists of SiO2. Silicon dioxide is furthermore well suited to compensating for the temperature response of the elastic components of the substrate.
In one embodiment, the electroacoustic transducer is a GBAW component. In a GBAW component, acoustic waves propagate at a boundary layer between a piezoelectric layer and a dielectric layer arranged thereon.
In one embodiment, the widths of the outer edge regions are determined by the lateral distance between the ends of the electrode fingers of one electrode and the other electrode, i.e., the busbar itself. In such an embodiment, the electroacoustic transducer has no stub fingers. Alternatively, stub fingers in the regions of the busbars are possible in order to suitably set the mass covering. Stub fingers are electrode fingers which do not overlap electrode fingers of the opposite electrode and therefore excite substantially no longitudinal acoustic waves.
The distance W, which corresponds to the distance between the central excitation region and the outer edge region in the case of convex slowness and to the distance between the central excitation region and the busbars in the case of concave slowness, can be
In this case, f is the operating frequency, ΔvAB=|vZAB−vAB|, ΔvRB=|vZAB−vRB| and vZAB is the velocity in the central excitation region.
In the case of a convex slowness, vRB is the longitudinal velocity in the inner edge region and vAB is the longitudinal velocity in the outer edge region.
In the case of a concave slowness, vRB is the velocity averaged over the inner edge region and the outer edge region. vAB is the velocity in the region of the busbar.
In one embodiment, the inner edge region is significantly wider (e.g., 2 times, 5 times or 10 times wider) than the outer edge region.
In the case of concave slowness, the width of the regions of the busbars can be greater than or equal to
In this case, kyAB is:
In the case of convex slowness, the width of the outer edge regions can be greater than or equal to
In this case, kyAB is:
In one embodiment, the widths of the outer edge regions are determined by the lateral distance between the ends of the electrode fingers of one electrode and the ends of stub fingers interconnected with the busbar of the other electrode.
In one embodiment, the electroacoustic transducer is part of a resonator that operates with acoustic waves, having reflectors that delimit the acoustic track in a longitudinal direction.
In a longitudinal direction, further electroacoustic transducers can also arrange between reflectors delimiting the acoustic track. In this case, one or more transducers can be input transducers which convert RF signals into acoustic waves, while one or more other transducers are output transducers which convert acoustic waves into RF signals.
In one embodiment, a resonator that operates with acoustic waves comprises reflectors which delimit the acoustic track in a longitudinal direction. In this case, at least one of the reflectors has the same transverse velocity profile of the acoustic wave as the transducer.
In one embodiment, the transducer is arranged together with a reflector on a piezoelectric substrate, wherein the reflector has reflector fingers having the same construction as the electrode fingers of the transducer in a transverse direction.
In one embodiment, the invention specifies an electroacoustic transducer arranged in an acoustic track. The transducer comprises a piezoelectric substrate and two electrodes arranged thereon and each having interdigital electrode fingers interconnected with a busbar, for the excitation of acoustic waves. The transducer is designed such that the acoustic wave in a plurality of regions running parallel to the acoustic track has a different longitudinal propagation velocity. The longitudinal propagation velocity is the velocity of the acoustic wave in a longitudinal direction.
The transducer comprises a central excitation region with a first longitudinal velocity. Outer edge regions flank the central excitation region. The velocity of the acoustic waves in the outer edge regions generally deviates from the longitudinal velocity in the central excitation region. Regions of the busbars flank the outer edge regions of the electroacoustic transducer. The longitudinal velocity is lower in the regions of the busbars of the electroacoustic transducer than in the outer edge regions. The substrate with electrode structures has a self-focusing slowness. The presence of a self-focusing slowness is equivalent to an anisotropy factor Γ of the substrate which is substantially equal to −1: Γ=−1.
Further features with which transducers operating with acoustic waves with improved mode generation can be obtained are specified below.
A corresponding transducer comprises a central excitation region, if appropriate inner edge regions flanking the central excitation region, if appropriate gap regions flanking the inner edge regions, if appropriate outer edge regions flanking the gap regions, and regions of the busbars which flank the gap regions or outer edge regions, if present. The gap regions can be characterized in that in them electrode fingers of at least one electrode are interrupted.
The longitudinal propagation velocities of the acoustic waves propagating in the transducer are set suitably in the different regions in order to be able to obtain, in particular, a piston mode. For this purpose, it is possible to optimally set, for example, the mass covering per wavelength λ, in inner edge regions, in gap regions, in outer edge regions or in regions of the busbars. The setting of the mass covering is possible in the case of substrates having convex slowness (Γ>−1), concave slowness (Γ<−1) or in the case of an anisotropy factor of Γ=−1. As a result, as far as possible the entire acoustic energy is utilized for the excitation of exclusively the desired fundamental mode, e.g., of the piston mode. As a consequence, this results in reduced dips in the filter transfer function of corresponding filter components and in reduced shear loads on the electrode fingers as a result of higher modes, as a result of which the performance of the device is improved.
In particular, in the case of substrates having convex slowness it is possible to increase the mass covering in the inner edge regions, and in the case of substrates having concave slowness it is possible to increase the mass covering in gap regions explained in greater detail below.
In the case of substrates having concave slowness it is also possible to increase the mass coverings in the outer edge regions and in the central excitation region.
In the case of substrates having concave slowness it is also possible to decrease the mass coverings in the inner edge regions, at least from a relative standpoint.
An increase in the mass covering can be achieved by one of the following measures:
Widening the fingers, i.e., locally increasing the metallization ratio η,
Increasing the finger thickness by applying a preferably heavier element, for example a metal or a dielectric, in the form of a weighting layer above the electrode layer, below the electrode layer or between layers of electrode layers configured in multilayered fashion,
Applying a continuous dielectric strip, here arranged parallel to the wave propagation direction, as weighting layer,
Applying a continuous strip composed of metal or a dielectric as weighting layer on a dielectric cover layer,
Reducing the mass covering in the remainder of the acoustic track, for example by selectively removing a dielectric cover layer or a weighting layer. In this case, a selective removal leads to a relative increase in the mass loading in those regions in which removal is not effected.
A, for example relative, decrease in the mass covering can be achieved by one of the following measures:
Reducing η, e.g., by reducing the finger width,
Increasing the mass covering in the rest of the acoustic track by one of the measures described above.
In the case of a substrate having convex slowness it is possible to increase the mass covering in inner edge regions. The inner edge regions can have a width of 0.1 to 3.0 in units of the wavelength λ of the longitudinal waves. The widths of the inner edge regions can be, in particular, between 0.25 and 1.0λ. The metallization ratio, η, in inner edge regions can be 0.9 or less η can vary over the inner edge regions.
Heavy metals, such as, for example, copper, gold, silver, platinum, tungsten, tantalum, palladium or molybdenum, or a heavy dielectric, such as, for example, tantalum oxide, for example Ta2O5, can be arranged in a weighting layer in the inner edge regions, for example on the electrode fingers. The layer thickness of such a weighting layer can be between 5% and 200% of the thickness of an electrode finger. Such a weighting layer can comprise one or more layer plies composed of individual elements or composed of an alloy. One or more adhesion layers for a better mechanical connection between the component and a weighting layer can comprise titanium. Such a weighting layer can also be arranged below the electrode fingers.
Such a weighting layer can, in particular, be arranged above the electrode fingers and consist of the same material as the electrode fingers. The thickness of such a weighting layer can be, in particular, 10% to 50% of the finger thickness.
The weighting layer can comprise a dielectric and cover the entire inner edge region. By means of a dielectric, the electrode fingers are not short-circuited. Such a weighting layer composed of Ta2O5 can have, for example, a layer thickness of 5% to 200% of the electrode finger thickness.
Above the electrode fingers, a dielectric insulation layer can be arranged in the inner edge regions. A weighting layer having a thickness of 10 nm to 1 μm can be arranged thereabove in the entire inner edge region.
Alternatively or as an additional measure, in all other regions outside the inner edge regions a dielectric layer, for example a compensation layer composed of SiO2, can be thinned, but at most to a thickness of 10% of the thickness in the inner edge regions.
The gap regions of a transducer can be a width of between 0.5 and 5.0λ. Outer edge regions flanking the gap regions can comprise stub fingers interconnected with the corresponding busbar. Such an outer edge region can in each case have a width of 1.0 to 5.0λ.
The width of the gap regions can be, in particular, 0.5 to 5.0λ. The outer edge regions can then be free of stub fingers.
The metallization ratio η can be 0.9 or less in the outer edge regions. In the case of such a high metallization ratio η, the ohmic losses of the electrode fingers are reduced.
The electrode fingers in the outer edge regions can be thickened by a weighting layer, which, for example, is a layer thickness of 5% to 200% in units of the finger thickness.
The electroacoustic excitation in inner edge regions can be reduced, for example, by phase weighting. For this purpose, the excitation centers, the centers between finger edges of adjacent fingers, can be displaced in a longitudinal direction. The displacement can be periodic or random. The displacement of the finger centers can be 0.25 in units of λ or less.
By suitably setting the electroacoustic excitation, it is possible to obtain an improved profile of the piston mode. For this purpose, the metallization ratio η can also be adapted periodically or in a randomly distributed manner η can be varied, for example, in a range of 0.1 to 0.9.
It is also possible to suitably set the mass covering in the outer edge regions. For this purpose, by way of example, the outer edge regions can comprise partial regions arranged alongside one another as seen in a longitudinal or transverse direction. Stub fingers can be arranged in an outer partial region of the outer edge regions. Inner partial regions of the outer edge regions which have no stub fingers can be arranged between the outer partial regions of the outer edge regions. The width of the gap regions can be between 0.1 and 1λ. The width of the inner partial regions of the outer excitation regions can be between 0.1 and 3.0λ and the width of the outer partial regions of the outer edge regions can be between 1.0λ and 5.0λ. In order to increase the mass covering in the outer edge regions, η can be increased to up to 0.9.
In general, the measures for increasing or for decreasing the velocity in one region with a set velocity can also contribute to increasing or to decreasing the velocity in other regions with a set velocity.
Outer edge regions, gap regions and inner edge regions arranged alongside one another can have a total width of 0.1λ to 3.0λ.
It is possible to increase only the mass covering in the gap region. The previous measures for increasing the mass covering in the inner edge regions or in the outer edge regions can be used as measures for increasing the mass covering in the gap region.
As substrates for components that operate with acoustic waves with an improved piston mode it is possible to employ piezoelectric substrates such as lithium niobate LiNbO3, for short: LN, or lithium tantalate LiTaO3, for short: LT.
Specifically, the substrates in the following table are possible, inter alia:
Abbreviation
Material
Euler angles
LN 128 Y-X
LiNbO3
(0°, 37.85°, 0°)
LN Y-X
LiNbO3
(0°, −90.0°, 0°)
LN 10 RY
LiNbO3
(0°, −80.0°, 0°)
LN 15 RY
LiNbO3
(0°, −75.0°, 0°)
LN Y-Z
LiNbO3
(0°, −90.0°, −90.0°)
LT 36 Y-X
LiTaO3
(0°, −54.0°, 0°)
LT 39 Y-X
LiTaO3
(0°, −51.0°, 0°)
LT 42 Y-X
LiTaO3
(0°, −48.0°, 0°)
Quartz 32
SiO2 (α-quartz)
(0°, 122.0°, 0°)
Quartz 36
SiO2 (α-quartz)
(0°, 126.0°, 0°)
Quartz 37.5
SiO2 (α-quartz)
(0°, 127.5°, 0°)
Quartz 39.5
SiO2 (α-quartz)
(0°, 129.5°, 0°)
Substrates having a deviation from the specified angles of up to a few tenths of a degree are also suitable.
In this case, the Euler angles are defined as follows: a set of axes x, y, z, which are the crystallographic axes of the substrate, are firstly taken as a basis.
The first angle, λ, specifies the amount by which the x-axis and the y-axis are rotated about the z-axis, the x-axis being rotated in the direction of the y-axis. A new set of axes x′, y′, z′ accordingly arises, where z=z′.
In a further rotation, the z′-axis and the y′-axis are rotated about the x′-axis by the angle μ. In this case, the y′-axis is rotated in the direction of the z′-axis. A new set of axes x″, y″, z″ accordingly arises, where x′=x″.
In a third rotation, the x″-axis and the y″-axis are rotated about the z″-axis by the angle θ. In this case, the x″-axis is rotated in the direction of the y″-axis. A third set of axes x′″, y′″, z′″ thus arises, where z″=z′″.
In this case, the x′″-axis and the y′″-axis are parallel to the surface of the substrate. The z′″-axis is the normal to the surface of the substrate. The x′″-axis specifies the propagation direction of the acoustic waves.
The definition is in accordance with the international standard IEC 62276, 2005-05, Annex A1.
By way of example, LiNbO3 having the Euler angles (λ=0°, μ=−75±15°, θ=0° is appropriate as GBAW or boundary acoustic wave components.
A transducer can comprise a metal having a higher density than aluminum, for example copper, gold, tungsten or an alloy of these metals, as main constituent.
Electrodes or the electrode fingers of a transducer can consist of a metal having a higher density than aluminum, for example copper, gold, tungsten or an alloy of these metals, as main constituent.
A compensation layer can be arranged on a transducer. A compensation layer can reduce or eliminate the temperature response of the frequency position of a component. Such a compensation layer can comprise SiO2, SiO, Al2O3 or SiOxNy. The thickness of such a compensation layer can be greater than or equal to 50% in units of λ.
By way of example, lithium niobate having a set of Euler angles of (λ=0°, μ=−90±3°, θ=0° is appropriate as an SAW substrate.
Electrodes can comprise a plurality of layers of individual elements or of different alloys. In particular, adhesion layers or barrier layers, which can reduce acoustomigration, can comprise titanium, titanium oxide or titanium nitride.
The total height of an electrode can be 4% to 7% in units of λ. The metallization ratio averaged over all edge regions and the central excitation region can be between 0.55 and 0.7; the finger period, which substantially determines the frequency response of the component, can be in the range of between 0.8 and 1.1 μm.
A compensation layer comprising planarized SiO2 can comprise a thickness of between 25% and 33% in units of λ.
A dielectric passivation or trimming layer can comprise silicon nitride. Such a passivation or trimming layer can comprise a thickness of less than 7% of the wavelength.
The angles (0°, 37.85±3°, 0°) are also appropriate as Euler angles for a piezoelectric lithium niobate substrate.
The total height of the electrodes can be between 6% and 8% in units of λ. The metallization ratio can be set to be between 0.5 and 0.65. The finger period can be between 1.8 and 2.1 μm.
The electrodes can be covered with a planar dielectric layer, e.g., comprising SiO2. The thickness thereof can be between 29% and 33% of the acoustic wavelength λ.
The electrodes or a dielectric layer arranged thereabove can be covered with an additional dielectric passivation or trimming layer comprising e.g., Si3N4 or SiO2. The thickness thereof can be up to 5% of the acoustic wavelength λ.
For one embodiment, lithium niobate substrates having the Euler angles (0°, 37.85±3°, 0°) are also appropriate.
The total height of the electrode can be between 6% and 12% of the wavelength. The metallization ratio can be between 0.5 and 0.58.
A corresponding transducer can be part of a duplexer for the WCDMA band II (1850-1990 MHz) and band III (1710-1880 MHz). For this purpose, the finger period defining the operating frequencies can be between 0.8-1.1 μm.
A compensation layer comprising silicon dioxide can have a thickness of between 30 and 50% λ.
A dielectric passivation or trimming layer can comprise silicon nitride and comprise a thickness of less than 7% in units of λ.
In one embodiment, the inner edge regions can be omitted and a good piston mode can nevertheless be obtained. For this purpose, transducers can comprise outer edge regions and gap regions as an alternative to inner edge regions. The number of regions having a set velocity is therefore not reduced. The gap regions can have a width of between 0.1 and 3.0λ. The outer edge regions can have a width of between 1.0 and 5.0λ.
In order to increase the metallization ratio, it is possible to increase the number of fingers per wavelength.
A dielectric weighting layer or a weighting layer composed of metal can have a thickness of 10 nm to 1 μm.
In one embodiment of a transducer, the mass covering is reduced in the inner edge regions in comparison with the outer edge regions or the gap regions. The total width of the inner edge regions and of the gap regions can be between 0.1 and 3.0λ.
A reduction of the mass covering can be achieved by a reduction of for example to values of greater than or equal to 0.1. A reduction of the mass covering can also be achieved by removing a dielectric cover layer to thicknesses of less than 1 μm.
Lithium tantalate having Euler angles of (0°, −48±7°, 0°) is appropriate as the substrate. Euler angles of (0°, 52≦μ≦−35°, 0°) are also possible.
The total height of the electrode fingers can be between 2.5 and 12% in units of λ. The metallization ratio η can be between 0.4 and 0.8. The finger period can be between 0.7 and 3.0 μm.
A transducer can comprise a dielectric passivation layer, comprising silicon nitride, for example, having a thickness of less than 2% of the wavelength.
The aperture of the acoustic track can have a width of between 10λ and 50λ. In particular, the aperture can be less than 20λ.
The width of the aperture and the width of the central excitation region, of the inner edge regions, of the gap regions, of the outer edge regions or of the regions of the busbars can be different for series resonators and parallel resonators and depend on the resonant frequency and the aperture. In particular, the mass coverings of different resonators can be configured differently.
A method for producing an electroacoustic transducer according to the invention comprises the steps of
One embodiment of the method comprises the steps of
The setting of the longitudinal velocities in transverse regions can be achieved by means of suitable material covering in the transverse regions. In this case, the velocity of acoustic waves is generally reduced by an increase in the mass on the substrate. The velocity is generally increased by a covering with material having high stiffness (e.g., Al2O3 or diamond). The choice of a suitable material of the mass covering therefore makes it possible both to increase and to decrease the velocity of the acoustic wave.
Electroacoustic transducers according to the invention are explained in greater detail below on the basis of exemplary embodiments and associated schematic figures.
In contrast thereto,
Outer edge regions ARB flank the inner edge regions IRB. The outer edge regions ARB do not actively participate in the conversion between RF signals and acoustic waves. However, acoustic waves are indeed capable of propagation in the outer edge regions ARB. On account of the reduced mass covering in the outer edge regions ARB, the longitudinal velocity in the outer edge regions ARB is increased compared with the longitudinal velocities of the inner edge regions, of the reflectors IRB and of the central excitation region ZAB.
The regions of the busbars SB in turn flank the outer excitation regions. The mass covering is maximal here, compared with the rest of the transverse regions; the longitudinal velocity is minimal.
The arising of transverse oscillation modes is a consequence of diffraction effects within the acoustic track having a finite width. The formation of a transverse profile according to the invention of the longitudinal velocity (piston mode) helps to reduce the arising of oscillation modes having a velocity in a transverse direction.
Curve D shows the admittance profile of an electroacoustic transducer in which the longitudinal velocities in the central excitation region, in inner edge regions, in outer edge regions and in the regions of the busbars are adapted for achieving a piston mode. Resonances occur at the same frequencies as in curve C; however, their amplitudes increase greatly only starting from approximately 25 MHz above the resonant frequency.
Curve E shows the calculated frequency-dependent admittance of an electroacoustic transducer whose longitudinal velocities in a central excitation region, in inner and outer edge regions and in regions of the busbar are adapted to a piston mode and in which the dispersion due to the anisotropy of Γ=−1 is excluded.
A further option for adapting the acoustic wave is so-called stub or dummy fingers which are arranged in the region of the busbars and are substantially opposite the ends of the electrode fingers of the respective other polarity.
A reduced velocity by comparison with the central excitation region ZAB is present in the inner edge region IRB, and an increased velocity in the outer edge region ARB. The outer edge region ARB serves here as a decay region in which the mode decays exponentially outwardly. On substrates with high coupling or with heavy electrodes, the configuration of the outer edge region ARB as a gap region TG is particularly advantageous since a large difference in velocity with respect to the central excitation region ZAB is obtained here by the omission of every second finger. The utilization of the gap region as a decay region is novel. The outer edge region ARB should have a width at least such that, at its outer edge, the amplitude of the mode has decreased to 10% of the value in the central excitation region ZAB. The inner edge region IRB serves here for adapting a quasi-linear profile of Ψ(y) in the central excitation region ZAB to the exponential profile in the outer edge region ARB. For this purpose, the width W is chosen suitably.
An electroacoustic transducer is not restricted to one of the exemplary embodiments described. Variations comprising, for example, further velocity ranges arranged in lateral regions or correspondingly shaped electrode fingers, or combinations of different embodiments, likewise constitute exemplary embodiments according to the invention.
Ruile, Werner, Mayer, Markus, Sauer, Wolfgang, Ebner, Thomas, Schmidhammer, Edgar, Rösler, Ulrike, Berek, Stefan, Eggs, Christoph, Hauser, Markus, Bleyl, Ingo, Wagner, Karl-Christian, Jakob, Michael
Patent | Priority | Assignee | Title |
10749498, | Sep 13 2016 | Murata Manufacturing Co., Ltd. | Elastic wave device, high-frequency front end circuit, and communication device |
10833649, | Apr 27 2016 | Kyocera Corporation | Acoustic wave element and communication apparatus |
11405020, | Nov 26 2020 | MURATA MANUFACTURING CO , LTD | Transversely-excited film bulk acoustic resonators with structures to reduce acoustic energy leakage |
11496116, | Jun 06 2017 | Murata Manufacturing Co., Ltd. | Acoustic wave filter device, multiplexer and composite filter device |
11611325, | Dec 19 2017 | Murata Manufacturing Co., Ltd. | Acoustic wave device |
11705883, | Oct 24 2019 | Skyworks Solutions, Inc | Acoustic wave resonator with mass loading strip for suppression of transverse mode |
11811392, | Oct 23 2019 | Skyworks Solutions, Inc | Surface acoustic wave resonator with suppressed transverse modes using selective dielectric removal |
11843363, | Jun 27 2016 | Murata Manufacturing Co., Ltd. | Elastic wave filter device |
11848658, | Oct 24 2019 | Skyworks Solutions, Inc | Acoustic wave resonator with mass loading strip for suppression of hyperbolic mode |
11870421, | Oct 23 2019 | Skyworks Solutions, Inc | Surface acoustic wave resonator with suppressed transverse modes using second bus bar |
9991873, | Mar 06 2013 | SNAPTRACK, INC | Microacoustic component and method for the production thereof |
Patent | Priority | Assignee | Title |
6617752, | Oct 27 2000 | MURATA MANUFACTURING CO , LTD | Surface acoustic wave device and manufacturing method therefor |
7453334, | Mar 21 2005 | TRIQUINT, INC | Leaky SAW resonator and method |
7538637, | Jul 10 2003 | SNAPTRACK, INC | Acoustic wave transducer with transverse mode suppression |
7576471, | Sep 28 2007 | Qorvo US, Inc | SAW filter operable in a piston mode |
20070018755, | |||
20080315972, | |||
20110068655, | |||
20130051588, | |||
DE102005061800, | |||
DE10331323, | |||
EP1871006, | |||
EP1962424, | |||
JP11101350, | |||
JP11298286, | |||
JP2000183681, | |||
JP2001267880, | |||
JP2002223143, | |||
JP2006217517, | |||
JP2007096527, | |||
JP2007507130, | |||
JP2009290472, | |||
JP2009521142, | |||
JP2010212543, | |||
JP2011101350, | |||
JP2013518455, | |||
JP5005958, | |||
JP715272, | |||
WO3038997, | |||
WO20070732722, |
Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
Sep 15 2010 | Epcos AG | (assignment on the face of the patent) | / | |||
Jul 17 2012 | JAKOB, MICHAEL | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 17 2012 | ROESLER, ULRIKE | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 18 2012 | EBNER, THOMAS | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 18 2012 | SAUER, WOLFGANG | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 18 2012 | WAGNER, KARL-CHRISTIAN | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 18 2012 | BLEYL, INGO | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 18 2012 | MAYER, MARKUS | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 18 2012 | RUILE, WERNER | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Jul 19 2012 | SCHMIDHAMMER, EDGAR | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Aug 21 2012 | EGGS, CHRISTOPH | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Aug 22 2012 | HAUSER, MARKUS | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Aug 27 2012 | BEREK, STEFAN | Epcos AG | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 029289 | /0210 | |
Feb 01 2017 | Epcos AG | SNAPTRACK, INC | ASSIGNMENT OF ASSIGNORS INTEREST SEE DOCUMENT FOR DETAILS | 041608 | /0145 |
Date | Maintenance Fee Events |
Jul 16 2019 | M1551: Payment of Maintenance Fee, 4th Year, Large Entity. |
Jul 13 2023 | M1552: Payment of Maintenance Fee, 8th Year, Large Entity. |
Date | Maintenance Schedule |
Feb 09 2019 | 4 years fee payment window open |
Aug 09 2019 | 6 months grace period start (w surcharge) |
Feb 09 2020 | patent expiry (for year 4) |
Feb 09 2022 | 2 years to revive unintentionally abandoned end. (for year 4) |
Feb 09 2023 | 8 years fee payment window open |
Aug 09 2023 | 6 months grace period start (w surcharge) |
Feb 09 2024 | patent expiry (for year 8) |
Feb 09 2026 | 2 years to revive unintentionally abandoned end. (for year 8) |
Feb 09 2027 | 12 years fee payment window open |
Aug 09 2027 | 6 months grace period start (w surcharge) |
Feb 09 2028 | patent expiry (for year 12) |
Feb 09 2030 | 2 years to revive unintentionally abandoned end. (for year 12) |